It has previously been proposed to use oxygen-rich aeration gas in covered aeration basins of activated sludge systems for reducing the biochemical oxygen demand (BOD) and/or the nitrogenous oxygen demand (NOD) of influent wastewater streams; BOD and/or NOD being hereinafter referred to as the oxygen demand. Such activated sludge systems have employed aeration basins in which the mixed liquor, composed of the influent wastewater and recycled sludge, is aerated in a series of stages formed by staging walls extending throughout the depth and width of the liquid flow path through the basin so as to form physically separated liquid stages. The geometry of each such liquid stage was selected, and each stage was aerated by a liquid-gas mixing device in such a manner that the mixed liquor had a substantially uniform oxygen demand throughout each liquid stage; this mode of operation of each stage being "completely mixed" as this term is used in the waste treatment art. Accordingly, the prior art systems utilized a series of such completely mixed liquid stages with the mixed liquor flowing from one completely mixed liquid stage to the next completely mixed liquid stage through very small, restricted openings in the liquid staging wall such that virtually no backmixing occurred; i.e., no mixed liquor from a downstream stage was permitted to flow backwards to an upstream stage.
Such prior art systems also staged the flow of the aeration gas by dividing the aeration basin into a plurality of gas stages which were physically separated from each other by the upper portion of the same wall forming the liquid staging wall, or alternatively, by a gas baffle extending from the cover of the basin into the uppermost layer of the mixed liquor. For example, one such gas and liquid staged system is taught in U.S. Pat. No. 3,547,815. Also, this general type of prior art system is illustrated in FIGS. 1 and 2 of the present drawings and will be further described hereinafter.
On the other hand, a substantially different and improved basin design and mode of operation has been discovered and is disclosed in copending application Ser. No. 414,770 now U.S. Pat. No. 3,954,606 which eliminates the need for liquid staging walls and permits the operation of the aeration basin, or portions thereof, in either a mode of operation which approaches the completely mixed mode known in the art as the CMAS mode, or which achieves a high degree of effective liquid staging by selectively varying the backflow ratio. That is, while the gas may be staged by the provision of gas baffles, the liquid flowpath through the basin is not obstructed by walls or partitions extending throughout the depth of the mixed liquor. Rather, the initial design of the basin and the aerator mixing factor N .times. D/U is maintained within certain ranges so as to control the backflow ratio and thereby control the degree of inter-zone mixing of the mixed liquor between adjacent liquid zones as more fully described in that application.
The present invention retains all of the advantages of the system and mode of operation disclosed in U.S. Pat. No. 3,954,606 and, in addition, enables the basin to be operated in a mode of operation in which the number of effective liquid stages is greater than the number of actual or effective liquid stages previously obtainable within a basin of a given length. As a result, the present invention enables the oxygen demand of the influent to be reduced to a lower level with the same or less horsepower previously required. Alternatively, the same degree of reduction of the oxygen demand; i.e., the same degree of purification may be achieved in an aeration basin of smaller volume than that previously required.
The present invention is based upon several different but interrelated discoveries which include the provision of staggered partitions extending partially across the width of the liquid flow path between aerators, the geometry of the liquid zones surrounding each aerator, the value of the aerator mixing factor which is utilized, and the size of the open, unobstructed portion of the liquid flow path between adjacent liquid zones.